Insoluble acidic salts of polyvalent metals:

The acidic salts of multivalent metals form one of the most extensively studies class of compounds. A huge range of compounds of this categories has been elaborates as ion exchangers. They involve phosphates, arsenates, antimonates, vanadates, molybdates, tungstates, tellurates etc. mostly of trivalent and tetravalent metals like Al(III), Cr(III), Fe(III), Ti(IV), Zr(IV), Sn(IV), Ce(IV) and Th(IV). These salts mostly act as cation exchangers and their exchange properties prominently arise from the presence of readily exchangeable hydrogen ions associated along with the anionic set. The bewildering array of these acidic salts is known, widely in the form of gels. Mixed acidic salts like zirconium arsenophosphate, tin (IV) arsenophosphate etc. have also been explored as ion exchangers.

Gels of these acidic salts have a potential of use for separation of heavy metal cations by column chromatography. They have been used for paper chromatography by impregnating the papers with them. The gels do not have a definite composition and are not very stable towards the hydrolysis of the acidic group. Because of uncertainity about the exact composition and structure of gels, it is very difficult to understand the exact mechanism of ion exchange reaction. A real breakthrough in these exchangers came when some of them were prepared in definite crystalline forms. Phosphates and arsenates of Zr(IV), Ti(IV), Sn(IV), Th(IV) and Ce(IV) have been obtained in crystalline forms. One of the most studied compounds of this series is zirconium phosphate which has been obtained with different degree of crystallinity and in different crystalline forms.